Automated Container Image Signing and Provenance

Implement signature verification to ensure only validated .NET images reach production.

In this lesson, you will learn how to bridge the gap between building a secure .NET image and ensuring only verified images are executed within your Kubernetes production environment.

The Core Concept: Trusting the Supply Chain

In a professional .NET ecosystem, building a container is only half the battle. If an attacker gains access to your container registry, they could push a malicious image with a valid tag (like latest), and your Kubernetes cluster would blindly pull and run it. Automated Image Signing and Provenance solve this by cryptographically "signing" your image during the CI/CD pipeline and enforcing a "policy check" at the cluster level.

Essentially, you are attaching a digital signature to your image that acts as a tamper-proof seal. When Kubernetes attempts to pull the image, an admission controller validates the signature against a public key. If the image hasn't been signed by your build server, the cluster will reject the deployment entirely.

Implementing Verification with Admission Controllers

To enforce this, we typically use tools like Cosign (from the Sigstore project) integrated into a Kubernetes Admission Controller (like Kyverno or OPA Gatekeeper). Instead of writing complex C# logic to verify images, we leverage declarative policies within our cluster.

A policy definition in a cluster might look like this:

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: verify-image-signature
spec:
  validationFailureAction: Enforce
  rules:
    - name: check-signature
      match:
        resources:
          kinds:
            - Pod
      verifyImages:
        - imageReferences:
            - "my-registry.azurecr.io/my-dotnet-service:*"
          attestors:
            - entries:
                - keys:
                    publicKeys: |-
                      -----BEGIN PUBLIC KEY-----
                      MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE...
                      -----END PUBLIC KEY-----

In this setup, your build pipeline signs the image after a successful docker build and integration test suite. By the time the deployment manifest reaches the cluster, the signature acts as the "final gatekeeper." Even if the .NET code itself is secure, this ensures that the infrastructure pipeline remains the sole source of truth for what executes in production.

Key Takeaway: Automated image signing transforms trust from an implicit assumption into a cryptographic requirement, ensuring that no unverified or tampered .NET binary can ever be instantiated in your production cluster.